Difference Between Active Filter and Passive Filter

Filters are a class of electronic circuits used in signal processing, to allow or block a desired signal range or a signal. Filters can be categorized at many levels based on the properties, such as active – passive, analog – digital, linear – non-linear, discrete time – continuous time, time invariant – time variant, and infinite impulse response – finite impulse response.

Active and passive filters are differentiated by the passivity of the components used in the filter circuit. If a component consumes power or incapable of power gain then it is known as a passive component. Components that are not passive are known as active components.

More about Passive Filters

Resistors, capacitors, and inductors all consume power when a current passes through them, and incapable of power gain; therefore, any RLC filter is a passive filter, especially with the inductors included. Another major characteristic of the passive filters is that the filters do not need an external power source for operation. The input impedance is low and the output impedance is high, allowing self-regulation of the voltages driving the loads.

Usually, in passive filters, load resistor is not isolated from the rest of the network; therefore, change in the load may affect the characteristics of the circuit and filtering process. However, there are no bandwidth restrictions for the passive filters, allowing satisfactory operation at very high frequencies. In lower frequency filters, the inductor used in the circuit tends to be larger, making the circuit bulkier. If higher quality and smaller size are required, the cost is increased significantly. Passive filters also create a little amount of noise, due to the thermal noise in the elements. Nevertheless, with proper design this noise amplitude can be minimized.

Since there is no signal gain, the signal amplification must be performed at a later stage. Sometimes buffer amplifiers may be needed to compensate the differences in the output circuit. .

More about Active Filters

Filters with components such as operational amplifiers, transistors, or other active elements are known as active filters. They use capacitors and resistors, but inductors are not used. Active filters require an external power source to operate because of the power consuming active elements in the design.

Since no inductors are used, the circuit is more compact and less heavy. Its input impedance is high and the output impedance is low, allowing to drive low impedance loads at the output. Generally, the load is isolated from the internal circuit; hence variation of the load does not affect the characteristics of the filter.

The output signal has a power gain, and the parameters like gain pass band and cutoff frequency can be adjusted. Several drawbacks are inherent to the active filters. The changes in the power supply may cause changes in the output signal magnitude and the high frequency ranges are limited by the active element properties. Also, feedback loops used for regulating the active components may contribute to oscillation and noise.

What is the difference between Active and Passive Filters?

• Passive filters consume the energy of the signal, but no power gain is available; while active filters have a power gain.

• Active filters require an external power supply, while passive filters operate only on the signal input.

• Only passive filters use inductors.

• Only active filters use elements kike op-amps and transistors, which are active elements.